Electromagnetic Acoustic Transducer Using Magnetic Shielding

ABSTRACT

A combined electromagnetic acoustic transducer (EMAT) is disclosed that uses magnetic shielding to increase the efficiency (defined as the received signal per unit of excitation current). In addition, electromagnetic shielding may also be used to reduce the direct coupling between the transmit and receive coils. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The disclosure relates generally to the field evaluating the integrityof bonds that adhere wellbore casing to a wellbore. More specifically,the present disclosure relates to a method and apparatus of producingand detecting acoustic forces within a wellbore casing to evaluate theintegrity of the casing.

2. Description of Related Art

As illustrated in FIG. 1 wellbores typically comprise casing 8 setwithin the wellbore 5, where the casing 8 is bonded to the wellbore byadding cement 9 within the annulus formed between the outer diameter ofthe casing 8 and the inner diameter of the wellbore 5. The cement bondnot only adheres to the casing 8 within the wellbore 5, but also servesto isolate adjacent zones (e.g. Z₁ and Z₂) within an earth formation 18.Isolating adjacent zones can be important when one of the zones containsoil or gas and the other zone includes a non-hydrocarbon fluid such aswater. Should the cement 9 surrounding the casing 8 be defective andfail to provide isolation of the adjacent zones, water or otherundesirable fluid can migrate into the hydrocarbon producing zone thusdiluting or contaminating the hydrocarbons within the producing zone,and increasing production costs, delaying production or inhibitingresource recovery.

To detect possible defective cement bonds, downhole tools 14 have beendeveloped for analyzing the integrity of the cement 9 bonding the casing8 to the wellbore 5. These downhole tools 14 are lowered into thewellbore 5 by wireline 10 in combination with a pulley 12 and typicallyinclude transducers 16 disposed on their outer surface formed to beacoustically coupled to the fluid in the borehole. These transducers 16are generally capable of emitting acoustic waves into the casing 8 andrecording the amplitude of the acoustic waves as they travel, orpropagate, across the casing 8. Characteristics of the cement bond, suchas its efficacy, integrity and adherence to the casing, can bedetermined by analyzing characteristics of the acoustic wave such asattenuation. Typically the transducers 16 are piezoelectric deviceshaving a piezoelectric crystal that converts electrical energy intomechanical vibrations or oscillations transmitting acoustic wave to thecasing 8. Piezoelectric devices typically couple to a casing 8 through acoupling medium found in the wellbore. Coupling mediums include liquidsthat are typically found in wellbores. When coupling mediums are presentbetween the piezoelectric device and the casing 8, they can communicatethe mechanical vibrations from the piezoelectric device to the casing 8.However, lower density fluids such as gas or air and high viscosityfluids such as some drilling mud may not provide adequate couplingbetween a piezoelectric device and the casing 8. Furthermore, thepresence of sludge, scale, or other like matter on the innercircumference of the casing 8 can detrimentally affect the efficacy of abond log acquired with a piezoelectric device. Thus for piezoelectricdevices to provide meaningful bond log results, they must cleanlycontact the inner surface of the casing 8 or be employed in wellbores,or wellbore zones, having liquid within the casing 8. Another drawbackfaced when employing piezoelectric devices for use in bond loggingoperations involves the limitation of variant waveforms produced bythese devices. Fluids required to couple the wave from the transducer tothe casing only conduct compressional waves, thus limiting the wavetypes that can be induced in or received from the casing. A great dealof information is derivable from variant acoustical waveforms that couldbe used in evaluating casing, casing bonds, and possibly even conditionsin the formation 18. Therefore, there exists a need to conduct bondlogging operations without the presence of a particular couplant. A needexists for a bond logging device capable of emitting and propagatinginto wellbore casing numerous types of waveforms, and recording thewaveforms.

Electromagnetic-acoustic transducers (EMATs) have been used innon-destructive testing. An EMAT acts through the following physicalprinciples. When a wire is placed near the surface of an electricallyconducting object and is driven by a current at the desired ultrasonicfrequency, eddy currents are induced in a near surface region of theobject. If a static magnetic field is also present, these eddy currentsexperience Lorentz forces. These forces cause an acoustic excitation inthe object. In a reciprocal use, an electric signal will be generated inthe wire as a result of acoustic excitation in a metal placed close to apermanent magnet. Attenuation and/or reflection of the acoustic wavesbear information on the defects and surroundings of the object. An EMATis typically designed to producing a single waveform, such as shearhorizontal waves (SH) or Lamb waves.

Various EMAT design configurations have been proposed. U.S. Pat. No.4,296,486 to Vasile discloses an EMAT including a source of magneticflux for establishing a static magnetic field, an electrical conductorfor conducting an alternating current in the static magnetic field, andan electrically conductive nonmagnetic shield disposes between thesource of magnetic flux and the conductor. U.S. Pat. No. 7,024,935 toPaige et al. discloses an EMAT including a magnetic unit arranged to bemoved relative to the material under test to magnetize a surface layerof the material, and an electrical winding supplied by an alternatingcurrent source, the magnetic unit and the electric winding, in use,being applied in sequence to the material under test whereby theelectrical winding is positioned adjacent the material subsequent tomagnetization thereof by the magnetic unit, the alternating magneticflux created by the winding interacting with the remnant magnetizationof the material to create ultrasonic vibration of the material.

As for all downhole applications, it would be desirable to increase thetransducer efficiency. In addition, as in most sensors using EMantennas, it would be desirable to reduce the effects oftransmitter-receiver coupling and the effects of the object beingexamined on the antenna impedance. The present disclosure addresses thisneed.

SUMMARY OF THE DISCLOSURE

One embodiment of the disclosure is an apparatus for evaluating atubular. The apparatus includes an electromagnetic coupling devicehaving a coil, a magnet, and a magnetic shield configured to be conveyedinto the tubular and couple acoustic energy within the tubular. Thecoupling device may be further configured to propagate an acoustic wavein the tubular. The coupling device may be further configured to recordan acoustic wave propagating in the tubular. The magnetic shield mayinclude an nonconductive soft magnetic material having a high saturationflux density and low radio frequency losses. The apparatus may furtherinclude an electromagnetic shield. The electromagnetic coupling devicemay be further configured to form a wave within the tubular, the wavehaving a polarization that is that of a compressional wave, a shearwave, a transversely polarized shear wave, a Lamb wave an/or a Rayleighwave. The tubular may be a casing in a wellbore and apparatus mayinclude a logging tool including the electromagnetic coupling device.The apparatus may include a plurality of the electromagnetic couplingdevices on the logging tool. The plurality of electromagnetic couplingdevices may be axially and/or circumferentially spaced apart. Theapparatus may further include a processor configured to determine avelocity of propagation of an acoustic wave in the tubular using asignal generated by one of the plurality of electromagnetic couplingdevices and received by another of the plurality of electromagneticcoupling devices.

Another embodiment is a method of evaluating a tubular. The methodincludes conveying an electromagnetic coupling device having a coil, amagnet, and a magnetic shield into the tubular, conveying an electricalcurrent to the coil, and coupling acoustic energy to the tubular.Coupling the acoustic energy may further include propagating an acousticwave in the tubular, or recording an acoustic wave propagating in thetubular. That method may further include using, as the magnetic shield,a soft magnetic material having a high saturation flux density and lowRF losses. The method may further include using an electromagneticshield. The method may further include using the electromagneticcoupling device to form a wave within the tubular, the wave having apolarization that is that of one of a compressional wave, a shear wave,a transversely polarized shear wave, a Lamb wave and/or a Rayleigh wave.The tubular may be a casing in the wellbore and method may furtherinclude disposing the electromagnetic coupling device on a logging tool.The method may further include using a plurality of electromagneticcoupling devices disposed on the logging tool. The method may furtherinclude positioning the plurality of electromagnetic coupling devicesaxially and/or circumferentially spaced apart. The method may furtherinclude determining a velocity of propagation of an acoustic wave in thetubular using a signal generated by one of the electromagnetic couplingdevices and received by another of the plurality of electromagneticcoupling devices.

Another embodiment is a computer-readable medium for use with anapparatus for evaluating a tubular. The apparatus includes anelectromagnetic coupling device comprising a coil, a magnet and amagnetic shield configured to be conveyed into the tubular and couplingacoustic energy within the tubular, and a source of electrical currentconfigured to convey current to the coil. The medium includesinstructions that enable at least one processor to use theelectromagnetic coupling device to produce a wave in the tubular havinga polarization that is that of a compressional wave, a shear wave, atransversely polarized shear wave, a Lamb wave, and/or a Rayleigh wave.The medium may include a ROM, an EPROM, an EEPROM, a flash memory,and/or an optical disk.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure and its advantages will be better understood byreferring to the following detailed description and the attacheddrawings in which:

FIG. 1 depicts a partial cross section of prior art downhole cement bondlog tool disposed within a wellbore;

FIG. 2 shows the field configuration of an EMAT;

FIG. 3( a)-(b) (prior art) shows a layout of a prior art transducer forshear wave generation;

FIGS. 4( a), 4(b) and 4(c) show results of 2-D simulation of thetransducer of FIG. 3 showing the isolines of the static magnetic field,the RF magnetic field, and the RF magnetic field with the assumption ofa non-conductive magnet;

FIG. 5 shows the layout of a modified transducer;

FIGS. 6( a) and 6(b) show results of 2-D simulation of the transducer ofFIG. 5 showing the isolines of the static magnetic field and the RFmagnetic field,

FIG. 7 shows the layout of a another embodiment of a shear wavetransducer;

FIGS. 8( a) and 8(b) show results of 2-D simulation of the transducer ofFIG. 7 showing the isolines of the static magnetic field and the RFmagnetic field,

FIG. 9 (prior art) hows a layout of a prior art transducer for Lamb wavegeneration;

FIGS. 10( a), 10(b) and 10(c) show results of 2-D simulation of thetransducer of FIG. 9 showing the isolines of the static magnetic field,the RF magnetic field, and the RF magnetic field with the assumption ofa non-conductive magnet;

FIG. 11 shows the layout of a modified Lamb wave transducer;

FIGS. 12( a) and 12(b) show results of 2-D simulation of the transducerof FIG. 11 showing the isolines of the static magnetic field and the RFmagnetic field,

FIG. 13 shows the layout of another embodiment of a Lamb wavetransducer;

FIGS. 14( a) and 14(b) show results of 2-D simulation of the transducerof FIG. 12 showing the isolines of the static magnetic field and the RFmagnetic field, and

FIGS. 15( a), 15(b) and 15(c) show the effect of an EM (conductiveshield) on the stray RF field that may result in cross-talk between thetransmit/receive wires: for no shielding, partial shielding and completeshielding respectively.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 2 shows basic geometry for the shear wave excitation based onLorenz force. A current i_(exc)(x,z)e^(iwt) indicated by 203 flows inthe x-direction proximate to a metal body 201. This induces a currentj_(exc)(x,z)e^(iwt) in the y direction in the metal which then producesa displacement u_(exc)(x,z)e^(iwt) in the metal. The three equationsbelow represent respectively the acoustic amplitude, the receiver signaland the transducer efficiency defined as receive signal per unit currentin the transmit wire:

$\begin{matrix}{{u \propto \frac{B_{0} \cdot B_{exc}}{d \cdot s}},{{for}\mspace{14mu} \delta {\operatorname{<<}\lambda_{a}}},} & (1) \\{{S_{r} \propto {u \cdot B_{0} \cdot \frac{B_{r}}{I_{r}}}},} & (2) \\{\frac{S_{r}}{I_{RF}} \propto {B_{0}^{2} \cdot \left( \frac{B_{r}}{I_{r}} \right) \cdot {\left( \frac{B_{exc}}{I_{exc}} \right).}}} & (3)\end{matrix}$

Here, u is the acoustic amplitude, B₀ is the static magnetic field,B_(exc) is the excitation magnetic field generating eddy currents in theobject, d is the density, s is the shear modulus,

$\frac{B_{r}}{I_{r}}$

is the receiver sensitivity function (defined based on reciprocityprinciple as the magnetic field generated magnetic field per unitcurrent),

$\frac{B_{exc}}{I_{exc}}$

is the magnetic field per unit current for the excitation coil (if thesame coil is used for transmit and receive modes of the transducer, then

$\left. {\frac{B_{r}}{I_{r}} = \frac{B_{exc}}{I_{exct}}} \right),$

δ is the electromagnetic skin depth in the object for excitationfrequency, and λ is the wavelength of the acoustic excitation. In whatfollows below, modeling results for different transducer configurationsare compared using static magnetic field flux density and the RFmagnetic field flux density per unit current as criteria.

FIG. 3( a)-(b) shows layout of a transducer for the shear wavegeneration. The acoustic wavelength is defined by the spatial period ofthe static magnetic field generation (magnetization directions of themagnets are shown by arrows in FIG. 3). A desired excitation mode isselected by choosing the operation frequency at which the transmitterwire is driven by the excitation current. The object being examined isdenoted by 201. The magnet is denoted by 205 with the directional arrowsindicating the polarity of the magnets. The back iron is denoted by 205and the transmit and receive wires by 203.

FIG. 4( a) shows the results of 2-D simulation for the configuration ofFIG. 3. Shown therein are the iso-field lines representing the staticmagnetic field. FIG. 4( b) shows the results of 2-D simulation with theiso-field lines representing the RF magnetic field of the transducer. Inorder to show the primary RF field only, the object conductivity was setzero at the excitation surface. FIG. 4( c) shows the results of 2-Dsimulation with the iso-field lines representing the RF magnetic fieldof the transducer with the assumption that the magnet is nonconductive.FIGS. 4( a)-4(c) also give scale for the display. Comparing FIG. 4( b)with 4(c), we see that the presence of a conductive magnet in FIG. 4( b)reduces the RF magnetic field by a factor of about 4. Sincenon-conductive magnets are typically one-third the strength ofconductive magnets, the use of a non-conductive magnet would causealmost 10 time reduction in B₀ ² term of the equation (3). Therefore,the following embodiments of the transducer are discussed.

FIG. 5 shows layout of a modified transducer. The major difference withthe transducer of FIG. 3 is the addition of soft magnetic shoes 507. Theshoes act as a magnetic shield concentrate RF flux resulted in boostingthe RF magnetic field at the object surface and shields the conductivemagnet from the RF field of the wire. FIG. 6( a) shows the results of2-D simulation for the configuration of FIG. 5. Shown therein are theiso-field lines representing the static magnetic field. FIG. 6( b) showsthe results of 2-D simulation with the iso-field lines representing theRF magnetic field of the transducer. Again, in order to show the primaryRF field only, the object conductivity was set zero at the excitationsurface. The modeling results show a slight reduction in the staticmagnetic field and a dramatic increase in the RF field per unit current.Compare

$\frac{\left( {70 - 60} \right)\mu \; T}{A}$

in FIG. 6( b) with

$\frac{\left( {10 - 15} \right)\; \mu \; T}{A}$

in FIG. 4( b).

It is important to note that the soft magnetic material to be used inthis application should possess a high saturation flux density and lowRF losses in the frequency range up to 1 MHz. In application to EMAT thesoft magnetic material may not have high magnetic permeability since theeffective permeability of the magnetic shoes is predominantly shapelimited. From a practical standpoint, the permeability of the materialcould be as low as 10-20. Use of this type of materials as an antennacore in a NMR sensor is disclosed in the U.S. Pat. No. 6,452,388 toReiderman et al.

FIG. 7 represents another embodiment of the shear wave EMAT. In additionto the magnetic shoes, a conductive high saturation magnetic member 709is added. The configurations of the magnetic fields (See FIGS. 8 a-8 b)as well as the effect of the non-conductive soft magnetic material arebasically the same as in the EMAT of the FIG. 5.

FIG. 9 shows a Lamb wave EMAT of prior art. Presented in the FIGS. 10 a,10 b and 10 c are the static magnetic field, the RF magnetic field, andthe RF magnetic field with a hypothetical nonconductive magnet.

A design using a magnetic shield layer made of the above specifiednon-conductive soft magnetic material is presented in the FIG. 11. Thepresence of the layer 507 significantly increases the RF efficiency.Compare FIG. 12 b with FIG. 10 b.

Another configuration for a Lamb wave EMAT using the specified abovenonconductive soft magnetic material is presented in FIG. 13. Thetransducer includes a yoke 1311 made of soft magnetic material. Thetransducer demonstrates better efficiency for both static magnetic fieldand RF magnetic field. Compare FIG. 14( a) with FIG. 10( a) and FIG. 14(b) with FIG. 10( b).

FIGS. 15( a), 15(b), 15(c) show the effect of a EM (conductive) shieldon the stray RF magnetic field that may be a source of cross-talk(direct coupling) between two EMATs: one used in transmit mode andanother used in receive mode. The cross-talk is most severe when theacoustic waves propagation parameters are measured with two closelyplaced EMATs (e.g. inside a tubular). The figures show positions of afew RF magnetic field iso-lines in three cases. In FIG. 15( a), there isno shield (in the model the conductivity of the shield is set to 0). InFIG. 15( b), only the central part of the shield is active (theconductivity of the side wings is set to 0). In FIG. 15( c), the effectof full shielding is shown. For simplicity the non-zero conductivity wasset to infinity. The effect of the shield is obvious: the same RF fieldis left much closer to the transducer compared to the “no shield”situation.

The disclosure above has been limited to specific types of wavesproduced in the tubular. This is not to be construed as a limitation,and the novel features of the present disclosure may be used with otherprior art devices to generate a compressional wave, a shear wave, atransversely polarized shear wave, a Lamb wave, and/or a Rayleigh wave.

In a practical device, the signals received by the receive wire that areindicative of the motion of the object being examined are processed by aprocessor that may be downhole or uphole. Fundamental to the processingis the determination of a velocity of propagation. This may be done bymaking measurements of a propagating wave using a second transducerdisplaced from the first transducer, or by making measurements withdifferent portions of the transducer discussed above. The transmittingand receiving transducers may be circumferentially disposed on theoutside of a logging tool and/or axially disposed on the outside of thelogging tool. Thus, it is possible to determine the velocity ofpropagation of various types of waves in axial and/or circumferentialdirections.

Implicit in the control and processing of the data is the use of acomputer program on a suitable machine readable medium that enables theprocessor to perform the control and processing. The machine readablemedium may include ROMs, EPROMs, EEPROMs, Flash Memories and Opticaldisks. Such a computer program may output the results of the processingto a suitable tangible medium. This may include a display device and/ora memory device.

1. An apparatus for evaluating a tubular, the apparatus comprising: (a)an electromagnetic coupling device comprising a coil, a magnet and amagnetic shield configured to be conveyed into the tubular and couplingacoustic energy within the tubular.
 2. The apparatus of claim 1, whereinthe coupling device is further configured to propagate an acoustic wavein the tubular.
 3. The apparatus of claim 1, wherein the coupling deviceis further configured to record an acoustic wave propagating in thetubular.
 4. The apparatus of claim 1 wherein the magnetic shieldcomprises a non-conductive soft magnetic material having a highsaturation flux density and low RF losses.
 5. The apparatus of claim 1further comprising an electromagnetic shield.
 6. The apparatus of claim1 wherein the electromagnetic coupling device is further configured toform a wave within the tubular, the wave having a polarization that isthat of at least one of (i) a compressional wave, (ii) a shear wave,(iii) a transversely polarized shear wave, (iv) a Lamb wave, and (v) aRayleigh wave.
 7. The apparatus of claim 1 wherein the tubular comprisesa casing in a wellbore, the apparatus further comprising a logging toolincluding the electromagnetic coupling device.
 8. The apparatus of claim7 further comprising a plurality of electromagnetic coupling devicesdisposed on the logging tool.
 9. The apparatus of claim 8 wherein theplurality of electromagnetic coupling devices are positioned at leastone of: (i) axially spaced apart, and (ii) circumferentially spacedapart.
 10. The apparatus of claim 8 further comprising a processorconfigured to determine a velocity of propagation of an acoustic wave inthe tubular using a signal generated by one of the plurality ofelectromagnetic coupling devices and received by another of theplurality of electromagnetic coupling devices.
 11. A method ofevaluating a tubular, the method comprising: (a) conveying anelectromagnetic (EMAT) coupling device comprising a coil, a magnet and amagnetic shield into the tubular; (b) conveying an electrical current tothe coil; and (c) coupling acoustic energy to the tubular.
 12. Themethod of claim 11 wherein coupling the acoustic energy furthercomprises propagating an acoustic wave in the tubular.
 13. The method ofclaim 11 wherein coupling the acoustic energy further comprisesrecording an acoustic wave propagating in the tubular.
 14. The method ofclaim 11 further comprising using, as the magnetic shield, a softmagnetic material having a high saturation flux density and low RFlosses.
 15. The method of claim 11 further comprising using anelectromagnetic shield.
 16. The method of claim 11 further comprisingusing the electromagnetic coupling device to form a wave within thetubular, the wave having a polarization that is that of at least one of(i) a compressional wave, (ii) a shear wave, (iii) a transverselypolarized shear wave, (iv) a Lamb wave, and (v) a Rayleigh wave.
 17. Themethod of claim 11 wherein the tubular comprises a casing in a wellbore,the method further comprising disposing the electromagnetic couplingdevice on a logging tool.
 18. The method of claim 17 further comprisingusing a plurality of electromagnetic coupling devices disposed on thelogging tool.
 19. The method of claim 18 further comprising positioningthe plurality of electromagnetic coupling devices on the logging tool inat least one of: (i) an axially spaced apart arrangement, and (ii) acircumferentially spaced apart arrangement.
 20. The method of claim 18further comprising determining a velocity of propagation of an acousticwave in the tubular using a signal generated by one of the plurality ofelectromagnetic coupling devices and received by another of theplurality of electromagnetic coupling devices.
 21. A computer-readablemedium for use with an apparatus for evaluating a tubular, the apparatuscomprising: (a) an electromagnetic coupling device comprising a coil, amagnet and a magnetic shield configured to be conveyed into the tubularand coupling acoustic energy within the tubular; and (b) a source ofelectrical current configured to convey current to the coil; the mediumcomprising instructions that enable at least one processor to: (c) usethe electromagnetic coupling device to produce a wave in the tubularhaving a polarization that is at least one of: (i) a compressional wave,(ii) a shear wave, (iii) a transversely polarized shear wave, (iv) aLamb wave, and (v) a Rayleigh wave.
 22. The medium of claim 21 furthercomprising at least one of: (i) a ROM, (ii) an EPROM, (iii) an EEPROM,(iv) a flash memory, and (v) an optical disk.